1. Field of the Invention
This invention relates to the formation of a beam or flow of precursors for advanced materials, which precursors can be collected for future use or used to make or treat advanced materials such as semiconductor layers, photovoltaic cells, solar cells, or the like. An aspect of this invention relates to the use of microwave energy to prepare hydrides and/or organometallic compounds from volatile or volatilizable molecular species such as molecular hydrogen or molecular hydrocarbons and a metal or metalloid. An aspect of this invention relates to the collection of the hydrides and/or organometallic compounds thus prepared, as well as the use of these hydrides or organometallic compounds to provide (e.g. by decomposition) a source of elemental metal or metalloid which can be reacted with a substrate or deposited upon a substrate, e.g. in the form of metallic or metalloid layers which are extremely thin. Still another aspect of this invention relates to substrates upon which the metal or metalloid has been deposited. Still another aspect of this invention relates to apparatus suitable for carrying out the objectives described above.
2. Description of the Prior Art
It has long been known that extremely thin films of metals or metalloids can be deposited upon substrates. If desired, the thin films can be atomic or molecular in thickness. The ability to deposit such extremely thin films of metals and metalloids (particularly semi-conducting elements such as silicon, germanium, and the like) is of major importance in the field of solar and photovoltaic cells and other advanced materials which require the presence of dopants or semiconductors or other electrically conductive species. A very wide variety of techniques has been developed over the last twenty to thirty years for achieving thin film deposition of the type described above. These techniques have been continually evolving and have recently culminated in the development of three highly sophisticated techniques, i.e. molecular beam epitaxy (MBE), organometallic vapor phase epitaxy (OMVPE), and most recently chemical beam epitaxy (CBE), also referred to by some researchers as atomic layer epitaxy or metalorganic molecular beam epitaxy. Extremely advanced electronic materials can be produced by these processes, and persons skilled in this art believe that these may be the best candidates from amongst a gamut of methods which have been tried over the years for compound semiconductor growth including: sealed-tube synthesis, chemical vapor deposition with halide transport, liquid phase epitaxy, and physical vapor deposition. These earlier methods, from a generation ago, formed the basis for the current generation of highly sophisticated methods. A principal advantage of the more highly sophisticated methods, as compared to the earlier generation of methods, relates to the ability to achieve amazing control over growth and materials properties. Nevertheless, all three of these highly sophisticated techniques have drawbacks.
The most important drawbacks of MBE include: the need for ultrahigh vacuum conditions (hence a very large capital investment), severe limits on throughput rates, and difficulties of source flux control. A typical ultrahigh vacuum used in MBE is on the order of 10.sup.-10 torr. A typical throughput rate amounts to less than 10.sup.11 molecules per square centimeter--orders of magnitude below molar quantities. For many of these reasons, MBE tends to be most effective when used in research rather than in commercial manufacturing, although some commercial use of MBE has been made.
The ultrahigh vacuum requirements of MBE can be avoided completely with OMVPE. Indeed, pressures in excess of 10 torr have been used in OMVPE processes. However, OMVPE has at least one major drawback of its own; the raw material fed to the process is extremely toxic and/or reactive. For example, some OMVPE processes make use of hydrides of metalloids of Group Va of the Periodic Table (AsH.sub.3, SbH.sub.3, etc.). These hydrides are poisonous and must be handled with extreme care.
CBE represents an attempt to combine the features of MBE and OMVPE. Unfortunately, one is still obliged to use very low pressures (as in MBE), but instead of molecular beams of elemental precursors effused from heated source bottles, molecular beams from organometallic and hydride precursors are utilized.
Accordingly, this invention seeks to move up to still another generation of thin film coating technology which lacks the drawbacks of MBE, OMVPE, and CBE.